Generate markdown rather than index files that are Jekyll friendly

[raphaelcohn]

At the moment, it is possible to override the pandoc options to generate markdown output files. However, these are not Jekyll friendly as Jekyll requires a YAML preamble for every file. Personally, I think this is a sub-optimal choice, although common (nanoc did the same at one time), as it confuses most text editors and combines content and meta-data in ways with side effects. Having maintained a large site in the past that used this approach, I can testify to it not scaling. That said, are their users who'd want this?